The present invention relates generally to the field of signal processing and more specifically to a digital process that emulates an analog oscillator and ultimately a self excited loop (SEL).
A digital process has been developed that emulates an analog oscillator and a self excited loop (SEL) that can be used for field control. An SEL, in its analog form, has been used for many years for accelerating cavity field control. In essence the SEL uses the cavity as a resonant circuit, much like a resonant (tank) circuit is used to build an oscillator. An oscillating resonant circuit can be forced to oscillate at different, but close, frequencies to resonance by applying a phase shift in the feedback path. This allows the circuit to be phased-locked to a master reference, which is crucial for multiple cavity accelerators. For phase and amplitude control the SEL must be forced to the master reference frequency, and feedback provided for in both dimensions. The novelty of the present design is in the way digital signal processing (DSP) is structured to emulate an analog system. This is the first time that we know of the digital SEL concept has been designed and demonstrated for field control of superconducting accelerating cavities.
Presently the CEBAF Accelerator at Jefferson Lab is a 6 GeV five pass electron accelerator consisting of two superconducting linacs joined by independent magnetic transport arcs. In the future it is intended to increase the energy to 12 GeV. This is to be accomplished by adding five new 98 MeV cryomodules to each linac (10 total). To control the RF for these new cryomodules a new Low Level RF (LLRF) system is being designed. Cavity field control is maintained digitally using an Altera FPGA, which contains the feedback algorithm. The system incorporates digital down conversion, using quadrature under-sampling at an IF frequency of 70 MHz. A VME based system is being used as a prototype but it is planned to migrate to a stand alone EPICS IOC using a PC104 module.
Three upgrade cryomodules have been produced consisting of a new 7-cell cavity design and incorporating a new PZT mechanism. These cryomodules are discussed further in “Improved Prototype Cryomodule for the CEBAF 12 GeV Upgrade”, by E. Daly, et al., PAC 2003 Proceedings, Portland, Oreg., which is incorporated herein by reference. Two of these upgrade modules have been tested and installed, one in the CEBAF accelerator and the other in the Jefferson Lab FEL. Initial LLRF testing was done last summer. In these tests digital LLRF control was demonstrated to meet field control requirements up to 16.7 MV/m. For these tests a simple Generator Driven Resonator (GDR) algorithm was used. Further discussion of these test results can be found in “High Gradient Operation with the CEBAF Upgrade RF Control System”, by C. Hovater et al., LINAC 2006 Proceedings, Knoxville, Tenn., which also is incorporated herein by reference.
Self Excited Loop
The self excited loop was first presented by J. R. Delayen in “Phase And Amplitude Stabilization of Superconducting Resonators”, PhD Thesis, California Institute of Technology, 1978, which is incorporated herein by reference, and has been used since for control of superconducting accelerator structures. The heavy ion accelerators at Argonne, JAEA Tokai, Canberra and the electron accelerator at Darmstadt both use an analog version of the SEL. It is principally used for structures operated continuous wave (CW), with external Q's above 1×107, but can be used for any resonant structure.
An SEL algorithm has one distinct advantage over a GDR, it immediately excites a cavity regardless of frequency. For a superconducting cavity (SC) that has a large Lorentz detuning coefficient this allows an RF system to recover a cavity to the operational gradient without having to tune the cavity. In a CW RF system this is optimum. Even if the cavity has been detuned many bandwidths, the frequency difference between the reference and the cavity can be obtained and the cavity quickly tuned back. This is accomplished without the use of hunting algorithms or employing a Numerically Controlled Oscillator (NCO) which GDR systems must use.
What is needed in the field is a digital self excited loop. The ideal system would allow implementation in an integrated circuit such as a full pin-grid array (FPGA).